1. Field of the Invention
This invention relates to a dynamic quantity sensor that detects a dynamic quantity such as acceleration based on a change in capacitance between a comb-shaped movable electrode and a comb-shaped fixed electrode.
2. Description of the Related Art
FIGS. 1 and 2 show this kind of dynamic quantity sensor. The dynamic quantity sensor is manufactured by performing well-known semiconductor manufacture techniques to a lamination substrate 10 such as a SOI (Silicon On Insulator) substrate that is composed of a first semiconductor substrate 11, a second semiconductor substrate 12 laminated with the first semiconductor substrate 11 with an insulating layer 13 interposed therebetween.
A groove 14 is formed in the second semiconductor layer 12, and an opening portion 13a is formed in the first semiconductor layer 11 and the insulation film 13. Accordingly, the first semiconductor layer 11 forms a frame-shaped base portion, and the second semiconductor layer 12 forms a beam structure.
The beam structure formed by the second semiconductor layer 12 is composed of a weight portion 210, which is supported by the base portion through beam portions 22 so that it can be displaced in a predetermined direction (direction Y indicated by an arrow in FIG. 1), and comb-shaped movable electrodes 240 protruding from the weight portion 210, and comb-shaped fixed electrodes 310, 320 protruding from the base portion 11 to face respective side faces of the comb-shaped movable electrodes 240 while defining a specific interval therebetween.
When a dynamic quantity is applied to the sensor, the weight portion 210 is displaced in the direction Y due to elastic forces of the beam portions 22 so as to change a capacitance in detection interval 40 defined between the movable electrodes 240 and the fixed electrodes 310, 320, and the dynamic quantity can be detected based on the change in capacitance in the detection interval 40.
An object of the present invention is to improve characteristics of a dynamic quantity sensor that has a pole-shaped movable electrode and a pole-shaped fixed electrode and detects a dynamic quantity based on a change in capacitance between the movable electrode and the fixed electrode.
According to a first aspect of the present invention, in a dynamic quantity sensor having a movable electrode and a fixed electrode defining a detection interval therebetween, at least one of the movable electrode and the fixed electrode has a tapered plane shape with a width that decreases from a root portion toward a tip portion of the one of the movable electrode and the fixed electrode. Accordingly, the one of the movable electrode and the fixed electrode can have an improved rigidity. Therefore, sticking caused by bending of the electrode can be prevented.
According to a second aspect of the present invention, a dynamic quantity is detected based on a change in capacitance of a detection interval between a fixed electrode and a movable electrode while applying, to the fixed electrode and the movable electrode, carrier wave signals having a carrier wave frequency. The carrier wave frequency is shifted from 1/n (n: positive integer) of a natural frequency of the fixed electrode when the fixed electrode performs flexural vibration. Preferably, the carrier wave frequency is shifted from 1/n (n: positive integer) of a natural frequency of the movable electrode when the movable electrode performs flexural vibration.
When the carrier wave frequency is shifted from 1/n of the natural frequencies of the fixed electrode and the movable electrodes, the fixed electrode and the movable electrode can be prevented from resonating to perform the flexural vibration due to the carrier wave signals, resulting in improved accuracy of sensor output.
According to a third aspect of the present invention, a carrier wave frequency of carrier wave signals applied for performing self-diagnosis of a dynamic quantity sensor is shifted from. 1/n (n: positive integer) of a natural frequency of a fixed electrode when the fixed electrode performs the flexural vibration. Preferably, the carrier wave frequency in the self-diagnosis is shifted from 1/n of a natural frequency of a movable electrode. Accordingly, the fixed electrode and the movable electrode are prevented from resonating to perform the flexural vibration in the self-diagnosis.
According to a fourth aspect of the present invention, at least one of a movable electrode and a fixed electrode has a root portion and a tip portion in a protruding direction thereof, the root portion having a rigidity larger than that of the tip portion. In this case, the one of the movable electrode and the fixed electrode can have an increased strength. Further, because the rigidity of the root portion is larger than that of the tip portion in the electrode, a natural frequency of the electrode can be easily made larger than a carrier wave frequency of carrier wave signals used for a normal operation or self-diagnosis of the sensor.